Pollution, disease, and climate change are pushing the limits of what coral reefs can withstand. But, despite those harsh conditions, some corals are thriving. Scientists are trying to understand how that's possible, and what they're learning could save these incredible ecosystems from extinction.
Transcript of this Episode
Sam: Late at night, all across the ocean, animals move out of their skeletons to feed, stretching their long, stinging tentacles to capture nearby prey and then reeling it in to devour. These animals are, believe it or not, corals.
Deboki: Even though corals are often less than half an inch across, to their even tinier prey—called zooplankton—they’re fierce predators. But to us, corals make up beautiful, scuba-trip-worthy ecosystems hugely important both for the environment and the world’s economy.
Sam: Unfortunately, like with many ecosystems on this planet, we’re killing coral reefs. Pollution, overfishing, disease, and global warming are pushing the limits of what they can withstand.
Deboki: But some corals are surviving, even thriving, despite disease and warming temperatures, and scientists are trying to figure out how to save these incredible animals.
Sam: Which, by the way, were around for hundreds of millions of years before us humans came on the scene.
Welcome to Tiny Matters, a science podcast about things small in size but big in impact. I’m Sam Jones.
Deboki: And I’m Deboki Chakravarti. Today on the show, we’re talking about corals—what they are, why they’re so important, and how scientists are working to create long and short-term strategies to keep them around, hopefully, you know, for another 100 million years or so.
So let's start off with a little coral 101. Like Sam mentioned earlier, corals are animals. And most structures that we call "coral" are actually hundreds or thousands of tiny, soft-bodied creatures called coral polyps, which secrete a hard outer skeleton made of calcium carbonate that attaches either to rock or to the skeletons of other coral polyps.
Sam: With most types of coral, the polyps grow, die, grow, die, on repeat, laying a rocky-looking calcium carbonate foundation that creates the typical reef colonies you might be imagining—think Finding Nemo. But there are thousands of coral species and they can look like a lot of things—beautiful flowing fans, mushrooms, even squiggle-covered brains.
Deboki: And many corals, including the reef-building ones, contain photosynthetic algae called zooxanthellae. The coral and algae have a symbiotic relationship—the coral provides a protected environment as well as carbon dioxide and ammonium, which the algae need for photosynthesis. And, in return, the algae produce oxygen and carbohydrates that the coral uses for food. Plus they give corals a lot of their color.
Sam: A beautiful friendship.
Deboki: It really is. But it’s one that has been put under a lot of stress, especially over the last few decades. From 1957 to 2007, the amount of living coral in reefs was slashed in half.
Blake Ushijima: I was born and raised in Hawaii, so the marine ecosystem—very important. Fisheries, tourism, you know, daily life and culture. We spent a lot of time at the beach. I remember a very specific beach I used to go to because it had a very beautiful coral reef there, and from age six to 10, it disappeared just from development and runoff, and now it's just a mud flat.
Sam: That’s Blake Ushijima, a microbiologist at the University of North Carolina Wilmington. For Blake, seeing a change that dramatic in the waters where he grew up had a huge impact.
Blake: So within my childhood, I've seen the destruction of coral reefs. That's kind of one of the driving things that gets me out of bed every day is like, well, I don't want to see that anymore. I want us to do something to stop that.
Coral reefs are actually very important, even though they're estimated to take up only about 1% of the total ocean floor area, they can contain as much as 25% of all species. It’s a very diverse ecosystem. They are kind of like the trees of a forest—you know, they provide the physical structure, but also, you know, they are primary producers. So you can imagine if you don't have corals, it's like cutting down a forest.
Deboki: Corals are essential for the survival of a huge range of species, and many reefs are no longer able to support that diversity. In addition to all of that, coral reefs are keeping many of the world’s economies afloat.
Blake: There are nearly a hundred different countries and territories that depend on coral reefs or the Marine ecosystem. But besides that you have these multi-billion dollar industries, the fisheries, the tourism industry for any coastal city. In addition to that, you can have the physical attributes of coral reefs. A lot of people don't realize that coral reefs are very important for preventing erosion and coastal flooding, because it really breaks the energy from these storm surges and reduces the amount of coastal flooding.
Sam: So now let’s get into what’s destroying corals—in more detail than just “we are destroying them.”
Deboki: Unfortunately, we are destroying them in a lot of ways, with practices like overfishing, where we deplete reefs of certain species, completely throwing off the important prey/predator balance. There’s also runoff—things like sewage, pesticides, and pollutants like gasoline soak into the ground or get swept down storm drains and out into the sea.
Sam: But maybe the biggest threat to corals, both in the short and long-term, is global warming. As carbon dioxide builds in our atmosphere, turning our planet into a greenhouse, our oceans are absorbing a lot of that heat. As ocean temperatures rise, the corals’ symbiotic algae—the zooxanthellae that Deboki mentioned earlier—start producing reactive oxygen species. Reactive oxygen species fall under the category of “free radicals”—which is a term you might have heard of before. Free radicals have an unpaired electron and that makes them highly reactive.
Deboki: They do things like create breaks in DNA and inactivate proteins, damaging both the algae and the coral polyps. To protect themselves from that damage, the corals expel the algae. This process is called “coral bleaching” because without their colorful algae the coral are left looking, well, bleached.
Sam: So the corals kinda say to the algae, “yeah no thanks, I’d rather be nutrient-depleted than deal with your toxicity” and then don’t answer its calls.
Deboki: Hah basically I’m glad we’re anthropomorphizing the coral’s friendship breakup with the algae. The good-ish news is that when a coral bleaches, it’s not dead, but it can take decades to recover. And, unfortunately, bleached coral is less hearty and more likely to die.
Sam: Yeah, so it’s not dead but that’s still pretty terrible. I actually have this very vivid memory from about a decade ago. My sister was living in Costa Rica and I went to visit her, and while I was there I went to get my scuba license. So I was all psyched to check out this coral reef and I showed up and this thing was like a ghost of a reef—completely white, very few fish swimming around. Actually, there were fish, but mostly lionfish, which are invasive, covered in venomous spines, and wipe out a bunch of reef species as soon as they show up. It was a real bummer, and the instructor training me told me how much the reef had changed in just the last few years.
Deboki: That sounds really striking and reminds me a lot of what Blake was saying about what he saw when he was a kid—how within just a few years what was once a beautiful coral reef had become a mudflat.
Sam: Absolutely. So in addition to coral bleaching, all of that CO2 in our atmosphere is making it harder for corals to create their calcium carbonate skeletons. As seawater absorbs carbon dioxide, it becomes more acidic, meaning it has more hydrogen ions floating around. And that leads to more bicarbonate and fewer carbonate ions being produced. So bicarbonate is just carbonate with a hydrogen bound to it, but that hydrogen prevents coral polyps from combining carbonate with calcium to create their skeletons. Some corals can work with bicarbonate, but most can’t.
Deboki: But Blake and his lab study a whole other issue that corals face: stony coral tissue loss disease.
Sam: Stony coral tissue loss disease was first reported in Florida in 2014. When corals become infected they develop white patches that quickly grow and take over. The most susceptible corals can die within months, sometimes weeks. Stony coral tissue loss has spread across Florida’s coast and a lot of the Caribbean. It’s now been found off the coast of at least 20 countries. And scientists are still trying to pin down what causes it.
Blake: Unfortunately for the field of coral disease, it's not as advanced as human disease. There've only been a few core pathogens discovered. One of the reasons is because, you know, it's, it's, it's hard to work on things that are in the ocean, you know, and you have, they're all sitting in seawater and it's basically a bacterial soup. So if you have these open lesions that are forming on these coral from infection, you have a lot of colonizers coming in and also the fact that corals are already covered with bacteria—you're trying to look for very specific microorganisms in this pool of millions of cells and figure out which one is pathogenic. In terms of where they come from, you know, they could be from the surrounding seawater, some of them could be part of the microbiome that typically doesn’t cause disease but under the right conditions it does.
Deboki: You've probably heard the word “microbiome” a lot in the past 5 years or so, but usually in the context of human health and not coral health. The human body is full of—and covered with—microbes like bacteria, fungi, and even viruses. Like tens of trillions of them.
Sam: And we want them there, usually, because they keep more dangerous microbes at bay. Same goes for coral. It’s mostly good that they have microbes living all over them. But when that microbe balance is thrown off, it can be devastating, which gets back to what Blake was saying about how a microbe causing stony coral tissue loss disease could be part of the coral’s microbiome. Maybe there’s just some shift that allows it to grow out of control. It’s like with humans and bacteria like staphylococcus, aka staph.
Deboki: Right. A lot of people carry staph bacteria, you and I probably are right now, but we’re fine. It’s when the staph is able to take over that things can get really dangerous, even deadly. It’s the same with corals. If the microbe balance is off, ones that cause stony coral tissue loss disease could take over.
Sam: In addition to possibly allowing for already-present microbes to become pathogenic, changes to a coral’s microbiome can make it susceptible to other invaders that might cause stony coral tissue loss disease. Some recent research has indicated that a virus might be responsible, or at the very least play an important role, but where that virus or another pathogen might come from is still a big question mark. Some researchers have hypothesized that it’s being spread by commercial shipping vessels—carried in the ships’ ballast water. Whatever’s causing it and however it’s getting there, it’s bad.
Blake: It's very widespread. So it’s over 100% of the Florida Reef Tract, spreading throughout the Caribbean. So it’s an unprecedented disease outbreak. You never haven't any coral disease like this before. But as bad as it sounds, not every coral colony dies. With any disease outbreak you do have individuals, in this case individual colonies, that survive or survive longer than others. And that's actually what we're most interested in because there's something intrinsically special about those colonies that allowed them to survive this disease outbreak. So we're actually looking through the microbiome of those surviving colonies and seeing if there's any particular microbes that are part of its microflora, that could be beneficial.
Deboki: Blake and his research group found a few promising microbes that they refer to as potential probiotics.
Sam: A probiotic is a live microbe that gives some sort of health benefit to its host, and improving or restoring the microbiota present is one means of doing that.
Deboki: Probiotics have taken off in the human health industry, but there’s still a lot of work that needs to be done to really evaluate how beneficial they are for our guts.
But we’re talking about corals today, and with diseased corals, Blake is seeing promising results using one probiotic in particular.
Blake: In the laboratory, when we treated diseased corals with it, it was able to stop disease on a majority of the coral fragments. And this was actually originally isolated from a disease-resistant great star coral. And the interesting thing is not only was it able to stop disease progression on a lot of the coral fragments, but it was also able to protect healthy corals from disease transmission.
Sam: Now, Blake’s working on a pilot study to bring the probiotic to Florida reefs.
Blake: You know, a lot of things work in the laboratory and not necessarily in the field. So we're actually right now with our Smithsonian collaborators and Nova Southeastern University we're working on the very first pilot studies of this potential probiotic in Florida reefs.
Sam: This is pretty exciting—I mean, it could prevent reefs from being decimated by this disease. There’s a lot of hope here, which Blake likes to remind people of.
Blake: There's a lot of troubled reefs. But like I tell my students and everybody, we shouldn't lose hope. I mean, as soon as you lose hope it makes the work a thousand times harder. And there are a lot of like-minded people, a lot of like-minded labs, that are trying to think of solutions and trying to change what's going on. There are still reefs out there—now it's our charge to protect what we have and to preserve them.
Deboki: The reefs off Florida and in the Caribbean—that Blake and other researchers are working hard to save—are the same reefs that caught the eye of marine biologist Emma Camp a couple decades ago.
Emma Camp: So I always had a love for the ocean and it's surprising cause I grew up in Essex, England. We don't have coral reefs, but I was lucky enough that my family took me on holiday when I was between eight, nine years old over to the Caribbean and I got to snorkel for the first time. And that's when I saw a coral reef. As a young child, seeing this underwater city that you can't see from above the surface, then when I looked underneath and there's all these fish and all of this marine life, that was when I fell in love with reefs.
Sam: Now, Emma is at the University of Technology Sydney, studying the largest coral reef in the world—the Great Barrier Reef.
Emma: The Great Barriers Reef’s about the size of Italy, the country. So when we think about that, there are parts of it that are severely degraded and there are parts that are still amazing, and that's why we get quite conflicted information about what the state is, because there are areas that are still thriving and look brilliant. But in the big picture, we know from the 1980s, that coral cover’s been declining. And we know in the last five to six years, we've lost about 50% of the coral cover that remains. So overall there's about 30 to 50% of the reef that still has live coral cover, which is a pretty, you know, desperate state to be in. But importantly there is that other percent there, that is why researchers like myself are still working so hard to ensure that that remains.
The Great Barrier Reef hasn't been lost, it's not, you know, dead by any means, but it is under immense threat. And what we do over the next decade is gonna determine how it is into the future and if it remains.
Sam: Emma is interested in understanding how some corals are surviving under harsh conditions caused by climate change.
Emma: During my PhD, I was in the Caribbean again and my project was to look at seagrass beds to see if they could buffer the impacts of ocean acidification. About a year in I realized that, because of the daily dynamics in these systems, it wasn't going to provide the buffering capacity that we predicted. And so that led me to ask, okay, well, what are the other reef-associated systems where we can find corals, but we don't normally find corals? And what is their biochemistry? What are their conditions? And could they provide some services that we just haven't considered for reefs? So this led me to looking in some pretty hostile environments, things like mangrove lagoons in particular, where actually the conditions are often warmer, more acidic, and have low oxygen. And they're the trio stresses that we are predicting and are seeing intensify for reefs under climate change. So it was then kind of this, this ‘aha’ moment that these systems are actually like a natural laboratory for us to consider how some corals are actually able to survive.
Deboki: Emma is looking at how well these corals are growing, how well their symbiotic algae are photosynthesizing, and using genetics to identify those species of symbiotic algae. She’s asking, “Is there anything these corals are giving up in order to survive in these environments?”
Emma: In some of the mangrove systems around the Great Barrier Reef, we found several coral species living in conditions that we're not predicting till the year 2100. So it's amazing that they're surviving there. Now the flip side of that is that they grow a lot slower and they're a lot smaller. So that's obviously that trade off that we are talking about and that kind of cautioned optimism, but how they do that, we're still learning. But one of the things is that they have entirely different species of the microalgae living with them.
Sam: Emma and some of her colleagues think these different species of algae may have adapted to not produce toxic chemicals as the water warms. Remember the reactive oxygen species we mentioned earlier?
Emma: Whilst those algae are really important to provide a lot of resources, when conditions become stressful, they produce a toxic reactive oxygen species—that actually is why the coral has to expel them from the tissue because it flips from being a friend to the bad guy. So we actually think that in these extreme corals that maybe the algae don't produce as much, or there's just some way that it’s able to deal with it, so that's kind of the area where we now need to look to understand more.
Us as managers and scientists, understanding what individuals are and aren't resilient to is crucial to ensure that we can manage that system effectively. Understanding, is it the environment itself? Is it kind of a refuge that for some reason there's just cool waters that protect them? And so the corals themselves actually have no innate tolerance to temperature stress, they just got lucky because of where they're found, or is it that actually genetically they are really tough and that's something that we can utilize to try and make others tougher? So, again, understanding that I would say is the key to managing reefs in the future.
Sam: When it comes to collecting this info, Emma and her lab take a mix of approaches.
Emma: Sometimes we will go to these environments and study them in situ—so in the environment. So we have things like chambers that we can put over the coral. It's non-destructive so that we can sample how they are both synthesizing and calcifying in the environment. We'll take small fragments and often freeze them immediately, so we can then bring them back and look at the DNA and the genetics of those individuals. Other times we'll actually take small fragments, keep them alive and bring them for lab manipulation to see, for example, how far can we push them? What is their upper thermal tolerance?
So the research is trying to understand the mechanisms that support stress tolerance, but the ultimate goal is that that knowledge can then be embedded in the variety of active management solutions that are currently being tried for reefs. So this could be anything from actually moving some of those extreme corals to a reef environment that’s degraded, growing them in a nursery and out-planting them to just build up tolerance. That's maybe the easiest and most simple way that they could be deployed at a local scale.
Deboki: Emma says the goal of this work is to build coral reef resilience in the short term. To deal with the long-term, changes around the globe need to happen immediately. I mean, they really needed to have happened decades ago, but here we are.
Emma: Action isn't happening quickly enough on climate change. And so that's where myself and several researchers globally are then seeing what are the other tools that we can implement to buy time for the reef whilst we get where we need to get to with climate action.
Sam: Near the end of our conversation, I shared with Emma what we had chatted about with Blake, and the work he and his lab are doing. And she was really enthusiastic about the combination of his research approaches and her own approaches in addressing coral health both in the short and long-term.
Emma: Ultimately both of them are looking at resilience, right? This idea that corals can be resilient to a stress, whether or not it's a disease outbreak, which, you know, you could think of as kind of a pulse stress, something that happens in the short term, but can be, you know, catastrophic for everything that's there. And then you also need resilience to those long term stresses that are, you know, potentially kind of slow burns but are compromising survival nonetheless. So the two are a hundred percent complimentary. It doesn't help you either way if you've got resilient corals to future climate change, but they all die in a disease outbreak. So the two very much need to go hand in hand.
Now what's interesting is that arguably if we can manage the stresses that cause these disease outbreaks, that's often easier for us to actually deal with than trying to deal with climate change, because that's gonna rely on global collaboration on reducing emissions. But I also would argue that there's probably a link to changing temperatures and other things that feed back into the disease outbreaks that ultimately will find a cause back to climate change.
Deboki: And for people who may be listening to this episode and thinking, “I live nowhere near a reef, this has nothing to do with me,'' Emma says “think again.”
Emma: We are all connected to the reefs and to our environment. And even if we live somewhere that isn't next to a reef, our actions feed back onto nature. We don't have to go to the Great Barrier Reef, we don't have to go to the Great Lakes, we don't have to go to the Amazon to have an impact on it. We're absolutely connected to the environment. We are part of the environment. And I think we've lost that mindset a little bit through industrialization—that we are somehow removed from nature. And I think if COVID has taught us anything, it should be that we are so inextricably linked to the health of the environment for our own health and wellbeing.
Deboki: Thanks for listening to Tiny Matters, a production of the American Chemical Society, a non-profit scientific organization based in Washington, DC. Tiny Matters is hosted by me, Deboki Chakravarti, and Sam Jones who is also our executive producer and audio editor.
Sam: This week’s script was edited by George Zaidan and was fact-checked by Michelle Boucher. The Tiny Matters theme and episode sound design are by Michael Simonelli, and our artwork was created by Derek Bressler. Thanks to Emma Camp and Blake Ushijima for chatting with us.
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Deboki: And how we can keep exploring the tiny things that matter, and sharing what we learn with you.
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